Via Ha.ckers.org, we get news of a cross-domain flaw using Flash or Silverlight content that allows the attacker to use the victim's browser as a proxy, including access to the user's session. Erlend Oftedal, the developer, explains how the system works and demonstrates the concept with a video. The flaw stems from developers lackadaisically allowing cross-domain requests from Flash across their whole domain (which obviously includes the user-account interactions); even Flickr and YouTube were culprits at one point.